Boat makes blunder entering fishery boundaries (5)
I believe the answer is:
ferry
'boat' is the definition.
(ferry is a kind of boat)
'blunder entering fishery boundaries' is the wordplay.
'blunder' becomes 'err' (I've seen this before).
'entering' is an insertion indicator.
'boundaries' means to remove the middle letters (only the edge or boundary letters remain).
'fishery' with its middle taken out is 'fy'.
'err' put into 'fy' is 'FERRY'.
'makes' acts as a link.
(Other definitions for ferry that I've seen before include "Passenger vessel operating on a regular schedule" , "Boat that plies from shore to shore" , "Scheduled passenger ship" , "Boat that provides transport over stretch of water" , "Boat across e.g. English Channel" .)